News of her casting was revealed by co-star Samuel L Jackson – who will reprise the role of Nick Fury in the blockbuster sequel – while speaking to the Wall Street Journal.

‘I think we begin shooting before March of next year,’ the actor said. ‘I know we’re shooting in London, that James Spader is Ultron and going to be the bad guy and that we added Ms Olsen.’

However Jackson did not comment on Olsen’s role in the film, saying: ‘I don’t know what she’s doing, if she’s on the inside or the outside. I haven’t seen a script.’

Reports surfaced last month that Joss Whedon was very keen for Olsen – who sprang to fame in the indie hit Marcy Martha May Marlene – to take the Scarlet Witch role after it was turned down by Saoirse Ronan.

Kick-Ass actor Aaron Taylor-Johnson – who stars alongside Olsen in the upcoming Godzilla remake – is also believed to be in the frame to play the character’s brother Quicksilver.

The brother-sister hero team are the children of Magneto, who is played by Sir Ian McKellen in the X-Men franchise.

Avengers Assemble stars Chris Hemsworth, Robert Downey Jr, Scarlett Johansson, Chris Evans and Jeremy Renner are all expected to return for the sequel.